The present invention relates to laser ablation microlithography and more generally to laser patterning using pulsed laser light. In particular, we disclose a new SLM design and patterning method that uses multiple mirrors per pixel to concentrate energy to an energy density that facilitates laser ablation, while keeping the energy density on the SLM mirror surface at a level that does not damage the mirrors. Multiple micro-mirrors can be reset at a very high frequency, far beyond current DMD devices made by and commercially available from Texas Instruments, TX. This design may usefully be combined with the writing pattern described in U.S. patent application Ser. No. 12/626,581.
Currently most lithography, e.g., for production of LCD displays, is done conventionally using a photoresist, which after exposure and development becomes an etch mask used to delineate a pattern in a uniform film deposited over the entire area prior to the patterning. This procedure ensures high quality, but involves many steps, and in some cases, most of the expensive material that has been blanket deposited goes to waste. Therefore, there is an active search for new processes that reduce the number of process steps and/or use less material. A number of such methods are known which use concentrated pulsed light to transfer material to the workpiece or modify the surface or a surface film. The exposure per pulse may be in range 1 to 5 J/cm2, although in other cases energy densities in the range of 0.1 to 1 J/cm2 may suffice.
It is known in the art to pattern photoresist and ablate films using masks. It is also known to use DMD devices instead of a mask. The throughput for DMD direct laser ablation is low because of the moderate power density and update rate that can be achieved with a DMD. By design, typical DMD mirrors act as individual modulators and have an area of 11×11 or 13×13 microns. The pulse power per mirror is limited partly by the size of the mirrors, partly by the risk of damaging or disturbing the function of the underlying CMOS or the function of the MEMS. DMD devices typically have a frame rate of 20-30 kHz, or up to about 70 kHz when only a small part of the chip area is updated. With limited frame rate and limited pulse energy, the average power is inadequate for many applications of ablation. It is known in the art to use Liquid Crystal on Silicon (“LCOS”) SLMs to control a multibeam ablation system. LCOS devices are slow in comparison, typically operating at 50-200 Hz and can possibly be optimized to operate in the low kilohertz range.
Ablation using an early model of TI's DMD is described in U.S. Pat. No. 5,208,818 (1993). A DMD of 1,000×1,000 pixel modulators is demagnified by a ratio of 85× onto an area 200×200 microns. An SLM produces an image that the system steps by 100 microns between laser pulses. Each modulator is separately driven and imaged from the DMD onto the substrate, with symmetrical demagnification. It is difficult to make many mirrors to contribute to one diffraction-limited laser spot on the workpiece, since the DMD is not designed for phase coherence from mirror to mirror. Therefore, the demagnification is at best one mirror area to one ablation spot. The technology disclosed can overcome such limits and allow high or extremely high energy density in the pulses which hit the workpiece.
It is known in the art to create a surface topography by laser ablation of the surface material. It is also known to blanket deposit a thin film, e.g. an organic, semiconducting, inorganic dielectric, or metallic film, and pattern it by laser ablation. It is further known to deposit a mask material, e.g. a polymer on top of the film, and pattern the mask material by laser ablation and use the remaining mask as an etch mask to pattern the film. Furthermore it is known to deposit a mask material, e.g. a polymer film, before the blanket deposition of the film and do a lift-off to pattern the film. Instead of the blanket deposition a selective deposition in certain areas may be done, e.g. by ink jetting, and the deposited areas may be trimmed by laser ablation, either directly or with the heat of an ablation-patterned etch or lift-off mask. It is further known to perform the ablation in air, inert gas, liquid, or vacuum. Ablation has been performed from the front side of the workpiece and from the backside of a transparent workpiece. US Patent Publication 2010/0141829 A1 mentions laser ablation using a powerful continuous-wave or pulse laser on top of a so-called spectral brush, primarily for medical applications identified as “laser therapy, laser cutting, and laser surgery.” Paragraph 0035. Spectral encoding of spatial information into back-reflection of the laser beam is an important feature of that publication. Paragraph 0016.
U.S. Pat. No. 6,423,925 (2002) describes laser peening of metal surfaces, which generates a shock wave as intense as 106 psi, thereby changing the metal surface. Col. 4, line 45. A spatial light modulator is mentioned, with the caution that the high fluence of the laser beam could be problematic in the selection and use of an SLM. Col. 6, line 59.
International Publication WO 2008/109618 A2 mentions laser ablation in the context of a substrate for holographic blank. Page 9. Polymers are identified as a good candidate for ablation material. Page 14. A state of the art description of Micro-Mirror Arrays is offered at page 20.
The disclosed methods and apparatuses are suitable for patterning by laser pulses, while the current application avoids damage to an SLM caused by high power concentrations on the SLM surface and allows improved throughput.